The dependence of periovulatory gonadotropin secretion on LHRH was assessed with the use of a potent LHRH antagonist [ALHRH; (Nac-L-Ala<sup>1</sup>, p-Cl-D-Phe<sup>2</sup>, D-Trp<sup>3,6</sup>) LHRH]. Blood samples were collected hourly from 14.00 h proestrus (P) through 09.00 h estrus (E) from intact cycling female rats. ALHRH was administered at 09.00 or 13.00 h Pbefore the proestrous increases in gonadotropins had commenced or at 23.00 h P after the LH and primary FSH surges had occurred but preceding the secondary FSH surge. Antagonist given at 09.00 or 13.00 h P completely blocked the LH release with levels remaining undetectable in most animals (<30 ng/ml) throughout the sampling period. However, administration of antagonist at these times failed to block completely the primary FSH surge although peak values were reduced when compared with controls, which displayed normal gonadotropin surges. In addition, ALHRH administered at 23.00 h failed to alter the magnitude or other characteristics of the secondary FSH surge when compared with controls.